Lots of naysayers here, but an advantage of protobuf is that proto files are hand-writeable, and therefore having an LSP for that could be useful.

That said, proto itself dissuades or forbids the kind of common things you might do with a LSP, such as renaming.

Renaming fields is a big no-no [edit: this isn't true, see corrections below], as is doing things like re-ordering fields.

A core idea of proto is that versions are strictly compatible with with previous versions. This itself has limitations and challenges for migrations, but encourages good practice about compatibility that usually gets ignored or hand-waved away in most ecosystems.

I accept however that it's often easy to offload both the re-structuring and the checking of version compatibility to an LLM and let them go at it.